1. Technical Field
The present invention relates to a lens holder and an optical pickup apparatus having the lens holder.
2. Description of the Related Art
Conventionally, widespread optical discs used for optically recording/reproducing information with an optical pickup apparatus include a CD medium using “780 to 790 nm” AlGaAs infrared laser for recording/reproducing and a DVD medium using “650 to 660 nm” AlGaInP red laser for recording/reproducing. As a result of widespread use, a general user has the CD medium and the DVD medium using laser lights with different wavelengths in hand, and therefore a CD/DVD compatible optical pickup apparatus is in widespread use to improve the level of convenience for a general user.
An important part of the CD/DVD compatible optical pickup apparatus is an optical system, especially, an objective lens responsible for forming a minute spot of laser light on the CD/DVD medium, and a wide variety of development is performed with a focus on the objective lens. For example, in a proposed mode, a CD medium objective lens and a DVD medium objective lens are disposed correspondingly to two laser lights in conformity with the CD/DVD standards in a CD/DVD compatible optical pickup apparatus and are switched depending on the CD/DVD medium placed on a turntable. And, in another proposed mode, one special objective lens compatible with both CD and DVD is disposed, which has a diffraction lens structure formed on an aspheric surface so as to correct spherical aberration caused by a difference between thicknesses of the protective layers of the CD/DVD medium.
Regarding the CD medium objective lens, the DVD medium objective lens, and the special objective lens compatible with CD/DVD, a single plastic lens is mainstream, which is advantageous in that the lens is lightweight, excellent in mass productivity, and has a higher degree of freedom of size/shape, as compared to a glass lens. By plastic molding, it is possible to design the CD medium objective lens and the DVD medium objective lens to be substantially the same in size/shape and in weight.
In the case of the mode of switching the CD medium objective lens and the DVD medium objective lens, a drive mechanism is needed, which performs complex feed control such that a optical axis of the CD/DVD medium objective lens corresponding to the placed medium is located on a geometric center line of the placed medium in every time the CD/DVD medium is placed on the turntable. Therefore, to eliminate such a complex drive mechanism, a method of disposing the CD/DVD medium objective lens in a lens holder (also known as optical pickup bobbin), is devised in some proposed technologies, wherein the lens holder is one of the components of the optical pickup apparatus.
For example, in the description of International Publication Pamphlet No. WO98/02874, as shown in FIG. 8A, a first objective lens 65 (e.g., CD medium objective lens) and a second objective lens 60 (e.g., DVD medium objective lens) are disposed in a lens holder 500, such that a line b-b′ linking between the optical axis P1 and the optical axis P2 becomes parallel to a tangent line c-c′ of an information track formed concentrically or spirally with a rotation center O as a base point on a surface of any one of different optical discs (52, 54) mounted on a turntable (not shown). As a result, the configuration of the optical pickup apparatus can be simplified, since the first objective lens 65 and the second objective lens 60 are driven in a focusing direction and a tracking direction by a common drive mechanism without switching a reference position.
In patent document 1 shown later, as shown in FIG. 8B, the first objective lens 65 and the second objective lens 60 are disposed in a lens holder 510 such that the optical axes P1, P2 are located on a line a-a′ passing through the rotation center O on a surface of any one of different optical discs (52, 54) mounted on a turntable. As a result, in either case of using the objective lenses 65 or 60, the information track can be highly accurately scanned with a light.
By the way, recently, a Blu-ray medium (hereinafter, referred to as BD medium) and an HD-DVD medium (hereinafter, referred to as HD medium) have been coming into practical use, which uses InGaN blue purple laser with a wavelength of “400 to 410 nm” for recording/reproducing. Therefore, an optical pickup apparatus has been developed, which has compatibility with the BD medium and/or HD medium in addition to that with the conventional CD medium and DVD medium. The following will be described on the premise that the optical pickup apparatus as mentioned above is exemplified by an optical pickup apparatus compatible with the BD medium, the CD medium, and the DVD medium (hereinafter, referred to as BD/CD/DVD compatible optical pickup apparatus).
With respect to the BD/CD/DVD compatible optical pickup apparatus, the optical system thereof must further be simplified in comparison with that of the CD/DVD compatible optical pickup apparatus since an optical system for the BD medium must newly be added, and the mode of disposing one special objective lens compatible with CD/DVD may be employed rather than the mode of disposing a CD medium objective lens and a DVD medium objective lens. As described above, a single “plastic lens” is mainstream regarding the special objective lens compatible with CD/DVD.
Since the BD medium objective lens (numerical aperture NA 0.85) has a high numerical aperture as compared to the CD medium objective lens (numerical aperture NA 0.45) and the DVD medium objective lens (numerical aperture NA 0.60), the BD/CD/DVD compatible optical pickup apparatus may employ a double lens including a combination of two plastic lenses or a single glass lens for ensuring a range of a working distance. However, since the manufacturing cost of the double lens is high and the working distance between the BD medium and the objective lens is as narrow as “0.15 mm”, the single “glass lens” is mainly employed to solve these problems. Currently, the practical use of the BD medium objective lens employing a single plastic lens is not yet within sight.
Therefore, it is conceivable that the mode in which the BD/CD/DVD compatible optical pickup apparatus employs the “glass lens” for the BD medium objective lens and the “plastic lens” for the CD/DVD compatible special objective lens will be mainstream in the future.
By the way, as shown in FIGS. 8A and 8B, in the case that the first objective lens 65 and the second objective lens 60 are substantially the same in size/shape and weight, the balance of the gravity centers can be easily maintained in the lens holders 500, 510 by arranging the first objective lens 65 and the second objective lens 60 symmetrically to the geometric centers X, Y of the lens holders 500, 510.
For example, in the case that the optical pickup apparatus covered by FIGS. 8A and 8B is the CD/DVD compatible optical pickup apparatus and also in the case that the first objective lens 65 is the CD medium objective lens and the second objective lens 60 is the DVD medium objective lens, the single plastic lenses which are substantially the same in size/shape and weight are mainly employed for the first objective lens 65 and the second objective lens 60. Accordingly, the balance of the gravity centers can be easily maintained in the lens holders 500, 510.
On the other hand, for example, in the case that the optical pickup apparatus covered by FIGS. 8A and 8B is the BD/CD/DVD compatible optical pickup apparatus and also in the case that the first objective lens 65 is the BD medium objective lens, i.e., a glass lens, and the second objective lens 60 is the CD/DVD medium objective lens, i.e., a plastic lens, the lens holder 500, 510 may have a problem that the balance of the gravity centers thereof is disrupted easily, when disposed with the first objective lens 65 and the second objective lens 60 as shown in FIGS. 8A and 8B. This is because the specific gravity of the glass lens is higher than that of the plastic lens and therefore the glass lens is twice or more heavier than the plastic lens even if the sizes/shapes thereof are substantially the same.
Due to the above, unnecessary resonances such as pitching and rolling are generated, and for this reason, balancers 501, 511 must be added in the direction of the first objective lens 65 and the second objective lens 60 disposed as shown in FIGS. 8A and 8B. When added with the balancers 501, 511, however, an objective lens drive apparatus 8 becomes heavier, which results in the difficulty of ensuring sensitivity to the drive mechanism. Since the number of components is increased by the number of those of the balancers 501, 511, a discrepancy between the drive point and the gravity center of the objective lens drive apparatus 8 is likely to be generated due to an error in weight and mounting position of each component, which results in a problem that unnecessary resonances may easily be generated.